Add retrieval evals (#7738)

docs/api_reference/evaluation.rst

docs/api_reference/index.rst

docs/core_modules/supporting_modules/evaluation/modules.md

docs/core_modules/supporting_modules/evaluation/root.md

...re_modules/supporting_modules/evaluation/usage_pattern.md

.../supporting_modules/evaluation/usage_pattern_retrieval.md

docs/examples/evaluation/QuestionGeneration.ipynb

docs/examples/evaluation/faithfulness_eval.ipynb

docs/examples/evaluation/retrieval/retriever_eval.ipynb

llama_index/evaluation/__init__.py

llama_index/evaluation/retrieval/__init__.py

llama_index/evaluation/retrieval/base.py

llama_index/evaluation/retrieval/evaluator.py

llama_index/evaluation/retrieval/metrics.py

llama_index/evaluation/retrieval/metrics_base.py